ORIGIN AND FORMATION OF SOILS. Ill 



Isomorphism. — In 1830, Mitscherlich, a Prussian phi- 

 losopher, discovered tliat a number of the elementary 

 bodies are capable o^ replacing each other in, combination^ 

 from the fact of their natur.d crystalline form being identic- 

 al ; they being, as he termed it, isomorphous^ or of like 

 shape. Thus, magnesia, lime, protoxide of iron, protoxide 

 of manganese, whicli are all protovide-hases^ form one 

 group, each of whose members may take the place of the 

 other. Alumina (Al.^ OJ and oxide of iron (Fe, O3) be- 

 long to another group of sesqui oxide-bases, one of which 

 may replace the other; while in certain combinations 

 silica and alumina replace each other as acids. 



These replacements, which may take place indefinitely 

 within certain limits, thus may greatly affect the composi- 

 tion without altering the constitution of a mineral. Of 

 the mineral amphibole, for example, there are known a 

 great number of varieties ; some pure Avhite in color, con- 

 taining, in addition to silica, magnesia and lime ; otliers 

 pale green, a small portion of magnesia being replaced by 

 protoxide of iron ; others black, containing alumina in 

 place of a portion of silica, and with oxides of iron and 

 m inganese in large jiroportion. All these varieties of 

 amphibole, however, admit of one expression of their 

 constitution, for the amount of oxygen in the bases, no 

 matter what they are, or what their proportions, bears a 

 constant relation to the oxygen of the silica (and alumina) 

 they contain, the ratio being 1 : 2. 



If the protoxides be grouped together under the gen 

 eral symbol MO (metallic i>rotoxide,) the composition of 

 the amphiboles may be expressed by the formula MO SiO,. 



In pyroxene the same r(>placements of j^rotoxide-bases 

 on the one hand, and of silica and alumina on the other, 

 occur in extreme range. (See analyses, p. 112.) The gen- 

 eral formula which includes all the varieties of pyroxene 

 is the same as that of ami)hibole. The distinction of am- 

 phibole from pyroxene is one of crystallization. 



